1. Field of the Invention
The present invention relates to the field of non-volatile memory devices. More particularly, the invention relates to a method of programming multi-bit flash electrically erasable programmable read only memory (EEPROM) cells that utilize the phenomena of hot electron injection to trap charge within a trapping dielectric material within the gate.
2. Discussion of Related Art
Memory devices for non-volatile storage of information are currently in widespread use today, being used in a myriad of applications. A few examples of non-volatile semiconductor memory include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash EEPROM.
Semiconductor EEPROM devices involve more complex processing and testing procedures than ROM, but have the advantage of electrical programming and erasing. Using EEPROM devices in circuitry permits in-circuit erasing and reprogramming of the device, a feat not possible with conventional EPROM memory. Flash EEPROMs are similar to EEPROMs in that memory cells can be programmed (i.e., written) and erased electrically but with the additional ability of erasing all memory cells at once, hence the term flash EEPROM.
An example of a single transistor Oxide-Nitrogen-Oxide (ONO) EEPROM device is disclosed in the technical article entitled xe2x80x9cA True Single-Transistor Oxide-Nitride-Oxide EEPROM Device,xe2x80x9d T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron Device Letters, March 1987. The memory cell is programmed by hot electron injection and the injected charges are stored in the oxide-nitride-oxide (ONO) layer of the device. This article teaches programming and reading in the forward direction. Thus, a wider charge trapping region is required to achieve a sufficiently large difference in threshold voltages between programmed and erased states, and only one physical location per cell is used to store the electrons.
An attempt to improve the programming of such ONO EEPROM devices by using two distinct locations per cell to store the electrons is disclosed in both U.S. Pat. No. 5,768,192 and PCT patent application publication WO 99/07000, the contents of which are hereby incorporated herein by reference. In those disclosed devices, a cell is programmed using hot electron programming, each bit is read in a direction opposite that in which it was programmed with a relatively low gate voltage. For example, the right bit is programmed conventionally by applying programming voltages to the gate and the drain while the source is grounded. Hot electrons are accelerated sufficiently to be injected into a region of the trapping dielectric layer near the drain. The device, however, is read in the opposite direction from which it was written, meaning voltages are applied to the gate and the source while the drain is grounded. The left bit is similarly programmed and read by swapping the functionality of source and drain terminals. Programming one the bits leaves the other bit with its information intact and undisturbed. Programming one of the bits, however, have a very small effect on the other bit, e.g., slightly slower programming speed for the second bit. However, the efficiency of such programming is often inadequate since a number of electrons are created by impact ionization in the substrate that are accelerated toward the center of the channel. Such xe2x80x9cspilloverxe2x80x9d electrons are undesirable because they are difficult to remove during the erasure cycle and they cause an increase in the threshold voltage of the complementary bit.
One aspect of the present invention regards a method of programming a memory cell with a substrate having a first region and a second region with a channel therebetween and a gate above the channel, and a charge trapping region that contains a first amount of charge. The method includes applying a constant first voltage across the gate and applying a second constant voltage across the first region. A third constant voltage is applied to the substrate so that spillover electrons are substantially reduced in number when compared with when the third constant voltage is absent.
A second aspect of the invention regards a method of programming a memory cell with a substrate having a first region and a second region with a channel therebetween and a gate above the channel, and a charge trapping region that contains a first amount of charge. The method includes applying a constant first voltage across the gate and applying a second constant voltage across the first region. Electric fields are reduced at the locations in the substrate where impact ionization occurs so as to substantially reduce the number of spillover electrons when compared with when the step of reducing of electric fields is not performed.
A third aspect of the present invention regards a memory cell that includes a substrate that has a first region and a second region with a channel therebetween. The memory cell further includes a gate above the channel and a charge trapping region that contains a first amount of charge, wherein the first region is doped to such an extent that electric fields are reduced at the locations in the substrate where impact ionization occurs during programming.
Each of the above aspects of the present invention provides the advantage of improving the efficiency of programming by decreasing the number of xe2x80x9cspilloverxe2x80x9d electrons that are created.
Each of the above aspects of the present invention provides the advantage of improving the endurance (cycling) of a memory cell.
The present invention, together with attendant objects and advantages, will be best understood with reference to the detailed description below in connection with the attached drawings.